Humidity control and method for thin film photovoltaic materials

ABSTRACT

A method for processing a thin film photovoltaic module. The method includes providing a plurality of substrates, each of the substrates having a first electrode layer and an overlying absorber layer composed of copper indium gallium selenide (CIGS) or copper indium selenide (CIS) material. The absorber material comprises a plurality of sodium bearing species. The method maintains the plurality of substrates in a controlled environment after formation of at least the absorber layer through one or more processes up to a lamination process. The controlled environment has a relative humidity of less than 10% and a temperature ranging from about 10 degrees Celsius to about 40 degrees Celsius. The method subjects the plurality of substrates to a liquid comprising water at a temperature from about 10 degrees Celsius to about 80 degrees Celsius to process the plurality of substrates after formation of the absorber layer. The plurality of substrates having the absorber layer is subjected to an environment having a relative humidity of greater than about 10% to a time period of less then four hours.

This application claims priority to U.S. patent application Ser. No.12/569,356, filed Sep. 29, 2009; which application claimed priority fromU.S. Provisional application No. 61/101,640, filed Sep. 30, 2008, bothcommonly assigned and incorporated by reference in their entirety hereinfor all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to photovoltaic materials andmanufacturing method. More particularly, the present invention providesa method and structure for fabricating a thin film solar cells. Merelyby way of example, the present method and structure include a thin filmwindow layer for manufacture of copper indium gallium diselenide basedthin film photovoltaic devices, but it would be recognized that theinvention may have other configurations.

From the beginning of time, mankind has been challenged to find way ofharnessing energy. Energy comes in the forms such as petrochemical,hydroelectric, nuclear, wind, biomass, solar, and more primitive formssuch as wood and coal. Over the past century, modern civilization hasrelied upon petrochemical energy as an important energy source.Petrochemical energy includes gas and oil. Gas includes lighter formssuch as butane and propane, commonly used to heat homes and serve asfuel for cooking Gas also includes gasoline, diesel, and jet fuel,commonly used for transportation purposes. Heavier forms ofpetrochemicals can also be used to heat homes in some places.Unfortunately, the supply of petrochemical fuel is limited andessentially fixed based upon the amount available on the planet Earth.Additionally, as more people use petroleum products in growing amounts,it is rapidly becoming a scarce resource, which will eventually becomedepleted over time.

More recently, environmentally clean and renewable sources of energyhave been desired. An example of a clean source of energy ishydroelectric power. Hydroelectric power is derived from electricgenerators driven by the flow of water produced by dams such as theHoover Dam in Nevada. The electric power generated is used to power alarge portion of the city of Los Angeles in Calif. Clean and renewablesources of energy also include wind, waves, biomass, and the like. Thatis, windmills convert wind energy into more useful forms of energy suchas electricity. Still other types of clean energy include solar energy.Specific details of solar energy can be found throughout the presentbackground and more particularly below.

Solar energy technology generally converts electromagnetic radiationfrom the sun to other useful forms of energy. These other forms ofenergy include thermal energy and electrical power. For electrical powerapplications, solar cells are often used. Although solar energy isenvironmentally clean and has been successful to a point, manylimitations remain to be resolved before it becomes widely usedthroughout the world. As an example, one type of solar cell usescrystalline materials, which are derived from semiconductor materialingots. These crystalline materials can be used to fabricateoptoelectronic devices that include photovoltaic and photodiode devicesthat convert electromagnetic radiation into electrical power. However,crystalline materials are often costly and difficult to make on a largescale. Additionally, devices made from such crystalline materials oftenhave low energy conversion efficiencies. Other types of solar cells use“thin film” technology to form a thin film of photosensitive material tobe used to convert electromagnetic radiation into electrical power.Similar limitations exist with the use of thin film technology in makingsolar cells. That is, efficiencies are often poor. Additionally, filmreliability is often poor and cannot be used for extensive periods oftime in conventional environmental applications. Often, thin films aredifficult to mechanically integrate with each other. Furthermore, thinfilms often degrade over time or have limitations in efficiency. Theseand other limitations of these conventional technologies can be foundthroughout the present specification and more particularly below.

BRIEF SUMMARY OF THE INVENTION

Embodiments according to the present invention relate to photovoltaicmaterials and manufacturing method. More particularly, the presentinvention provides a method and structure for fabricating a thin filmsolar cells. Merely by way of example, the present method and structureinclude a thin film window layer for manufacture of copper indiumgallium diselenide based thin film photovoltaic devices, but it would berecognized that the invention may have other configurations.

In a specific embodiment, a method for processing a thin filmphotovoltaic module is provided. The method includes providing aplurality of substrates, each of the substrates has a first electrodelayer and an overlying absorber layer composed of CIGS or CIS material.In a specific embodiment, the absorber material comprises a plurality ofsodium bearing species having a concentration of 5×10¹⁹ per cm³ andgreater in a specific embodiment. In a specific embodiment, the methodmaintains the plurality of substrates in a controlled environmentthrough one or more processes up to a lamination process or otherpackaging process. The controlled environment has a relative humidity ofless than 10% and a temperature ranging from about 10 Degree Celsius toabout 40 Degree Celsius in a specific embodiment. The method includessubjecting the plurality of substrates to a liquid comprising water at atemperature ranging from about 10 Degree Celsius to about 80 DegreeCelsius to process the substrate after formation of the absorber layer.The plurality of substrates having the absorber layer are subjected toan environment having a relative humidity greater than 10 percent for atime period of less than four hours, but can be others. Of course therecan be other variations, modifications, and alternatives.

In an alternative embodiment, a method for processing a thin filmphotovoltaic module is provided. The method includes providing asubstrate having a first electrode layer and an overlying absorber layercomposed of copper indium gallium selenide (CIGS) or copper indiumselenide (CIS) material. In a specific embodiment, the absorber materialincludes a plurality of sodium bearing species, the sodium bearingspecies has a concentration of about 5×10¹⁸ atoms per cm³ and greater.The method maintains the substrate in a controlled environment afterformation of at least the absorber layer through one or more processesup to a lamination process. The controlled environment is characterizedby a relative humidity of less than 10% and a temperature ranging fromabout 10 degrees Celsius to about 40 degrees Celsius in a specificembodiment. In a specific embodiment, the substrate is subjected to aliquid comprising water at a temperature from about 10 degrees Celsiusto about 80 degrees Celsius to process the plurality of substrates afterformation of the absorber layer. The method subjects the substratehaving the absorber layer to an environment having a relative humidityof greater than about 10% to a time period of less then four hours. Themethod further performs a lamination process on at least the substrateto form a solar module. The solar module is characterized by anefficiency parameter at a first efficiency level before the laminationprocess and a second efficiency level after the lamination process. Thefirst efficiency level is within a predetermined amount of the secondefficiency level in a specific embodiment. Of course there can be othervariations, modifications, and alternatives.

Many benefits can be achieved by ways of the present invention. Forexample, For example, embodiments according to the present provide aneasy to implement method to improve an conversion efficiency or lightextraction for a CIS or CIGS thin film photovoltaic cell. Additionally,the present method provides a cost effective way to fabricatephotovoltaic cells. Depending on the embodiment, one or more of thebenefits may be achieved. One skilled in the art would recognize othervariations, modifications, and alternatives. These and other benefitsare described throughout the present specification and more particularlybelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified process flow diagram illustrating a method forfabricating a photovoltaic cell according to an embodiment of thepresent invention.

FIG. 2-10 are simplified diagrams illustrating a method for fabricatinga photovoltaic cell according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments according to the present invention direct to fabrication ofphotovoltaic material. More particularly, embodiments according to thepresent invention provide a method and structure for fabricating a thinfilm photovoltaic cell. Merely by way of example, the present inventionprovides a method of treating a window layer to improve a light sockingcharacteristic and conversion efficiency of the photovoltaic cell. Butit would be recognize that embodiments according to the presentinvention would have a broader range of applicability.

FIG. 1 is a simplified process flow diagram illustrating a method offabricating a photovoltaic cell according to an embodiment of thepresent invention. The diagram is merely an example, which should notunduly limit the claims herein. One skilled in the art would recognizeother variations, modifications, and alternatives. As shown, the methodbegins with a Start step (Step 102). The method includes providing asubstrate (Step 104). The substrate includes a transparent substratematerial having a film stack comprises at least a first electrode layer,and an absorber layer, layer deposited thereon. The absorber layercomprises a copper indium selenide (CIS) material or a copper indiumgallium selenide (CIGS) material in a specific embodiment. In a specificembodiment, the substrate is maintained (Step 106) in a first controlledenvironment, which is desiccated, having a relative humidity of lessthan 10 percent and at a temperature ranging from about 10 DegreeCelsius to about 50 Degree Celsius. The substrate including at least theabsorber is removed form the controlled environment and subjected to anaqueous solution to deposit a window layer (Step 108). The aqueoussolution contains a cadmium species, an ammonia species and a thioureaspecies to form as a cadmium sulfide material for the window layer in aspecific embodiment. The substrate, now comprises the first electrodelayer, the absorber layer, and the window layer is allowed to be stored(Step 110) in a second controlled environment. The controlledenvironment is a desiccated environment characterized by a relativehumidity of less than 10 percent in a specific embodiment. The methodremoves the substrate and forming a second electrode layer (Step 110)overlying a surface region of the window layer. The second electrodelayer can be a transparent conductive oxide material such as indium tinoxide, or other doped oxides, depending on the application. Thesubstrate is stored in a third desiccated environment (Step 114). Themethod removes the substrate from the third desiccated environment andperforms a lamination process to seal the photovoltaic cell a specificembodiment. The method performs other steps (Step 118), including lightsoaking, testing, framing and others to form a solar module. The methodends with an end step (Step 120). Of course there can be othervariations, modifications, and alternatives.

The above sequence of steps provides a method of fabricating aphotovoltaic cell according to an embodiment of the present invention.More particularly, the present method provides a technique for improvingefficiency of the solar cell by environmental controlling themanufacturing and related methods. Depending on the embodiment, one ormore steps may be added, one or more steps may be eliminated, one ormore steps may be provided in a difference sequence without departingfrom the scope of the present invention. One skilled in the art wouldrecognize other variations, modifications, and alternatives.

FIGS. 2-11 are simplified diagrams illustrating a method of fabricatinga photovoltaic cell according to an embodiment of the present invention.These diagrams are merely examples and should not unduly limit theclaims herein. One skilled in the art would recognize other variations,modifications, and alternatives. As shown in FIG. 2, a transparentsubstrate member 202 including a surface region 204 is provided. Thesubstrate member can be a glass material in certain embodiment. In aspecific embodiment, soda lime glass is a cost effective option for thetransparent substrate member. Other suitable transparent substrates suchas quartz, fused silica, or solar glass can also be used. Each of thetransparent substrate can include a barrier layer deposited on a surfaceregion. The barrier layer prevents sodium ions from the glass materialto diffuse into photovoltaic material area in a specific embodiment. Thebarrier layer can be a dielectric material such as silicon oxidedeposited using technique such as a sputtering process, a chemical vapordeposition process, including plasma enhanced processes, and others.Other barrier materials may also be used. These barrier materialsinclude aluminum oxide, titanium nitride, silicon nitride, tantalumoxide, zirconium oxide depending on the embodiment.

Referring to FIG. 3, the method includes forming a first electrode layer302 overlying the surface region of the transparent substrate memberwhich can have a barrier layer formed thereon. The first electrode layermay be provided using a transparent conductor oxide (TCO) such as indiumtin oxide (commonly called ITO), fluorine doped tin oxide, and the like.In certain embodiments, the first electrode layer may be provided usinga metal material. The metal material may be molybdenum in a specificembodiment. Other suitable metal materials such as gold, silver,tungsten, nickel or an alloy may also be used. The first electrode layercan be formed using deposition techniques such as sputtering, plating,physical vapor deposition (including evaporation, sublimation), chemicalvapor deposition (including plasma enhanced processes) following by apatterning process in a specific embodiment. Of course there can beother variations, modifications, and alternatives.

As shown in FIG. 4, the method includes forming an absorber layer 402overlying a surface region of the first electrode layer. The absorberlayer can be a thin film semiconductor material in a specificembodiment. In a specific embodiment, the thin film semiconductormaterial is a p-type semiconductor material provided by a copper indiumdiselenide (CIS) material, or a copper indium gallium diselenide (CIGS)material, any combination of these, or others, depending on theapplication. Other materials such as copper indium disulfide, copperindium aluminum disulfide, copper indium gallium disulfide, and others,including any combinations. The absorber layer may be deposited bytechniques such as sputtering, plating, evaporation including aselenization step. Further details of the formation of the absorbermaterial made of copper indium disulfide may be found in U.S. patentapplication No. 61/059,253, titled “High Efficiency Photovoltaic Celland Manufacturing Method,” commonly assigned, and hereby incorporated byreference. Of course, there can be other variations, modifications, andalternatives.

As shown in FIG. 5, the method includes maintaining the substrate membercomprising the absorber layer in a first controlled environment 502 topreserve a conversion efficiency of the thin film photovoltaic. Thecontrolled environment is characterized by a relative humidity less than50 percent and preferably less than 10 percent in a specific embodiment.In a specific embodiment, the substrate member is exposed to a conditionhaving a relative humidity greater than about 10 percent for less thanabout four hours and preferably less than two hours. The conditionhaving a humidity greater than 10 percent is characterized by a watervapor species that has a high chemical activity. The controlledenvironment is further characterized by a temperature ranging from about10 Degree Celsius to about 40 Degree Celsius in a specific embodiment.In a specific embodiment, the copper indium diselenide or the copperindium gallium diselenide material can include a plurality of sodiumspecies having a concentration of about 5×10¹⁹ per cm³ or greater withina thickness of the copper indium diselenide or copper indium galliumdiselenide material and/or in a surface region of the copper indiumdiselenide or copper indium gallium diselenide material. For a yetunknown reason, the plurality of sodium species affect the performance,for example, conversion efficiency of the photovoltaic cell, uponexposure to a water vapor species, which can have a high chemicalactivity in a specific embodiment. The plurality of sodium species maybe a diffusion species from the first electrode layer or a compensatingspecies depending on the embodiment. The plurality of sodium species mayalso be added from an external source to improve the performance of thephotovoltaic cell in a specific embodiment. Of course there can be othervariations, modifications, and alternatives.

In a specific embodiment, the method forms a window layer 602 overlyinga surface region of the absorber layer. The window layer is oftenprovided using a wide bandgap n-type semiconductor material for a p-typeabsorber layer. For a copper indium diselenide or copper indium galliumdiselenide absorber material, the window layer can use a cadmium sulfidematerial in a specific embodiment. The cadmium sulfide material may bedeposited using techniques such as sputtering, vacuum evaporation,chemical bath deposition, among others. Of course there can be othervariations, modifications, and alternatives.

In a specific embodiment, the cadmium sulfide window material isdeposited using a chemical bath deposition method, which provides a costeffective way for a large area deposition and can be easily adapted fora batch process. The chemical bath deposition method uses an aqueoussolution comprising a cadmium species, a sulfur bearing species, and anammonia species in a specific embodiment. The sulfur bearing species canbe provided by a organosulfur such thiourea in a specific embodiment.The chemical bath deposition method can be provide at a temperatureranging from about 10 Degree Celsius to about 80 Degree Celsius in aspecific embodiment. The substrate including the window layer issubjected to a cleaning process, including one or more rinse and drysteps after deposition in a specific embodiment. The cleaning processcan use deionized water in a specific embodiment. In certainembodiments, other window materials may also be used. These other windowmaterials may include zinc sulfide (ZnS), zinc selenide (ZnSe), zincoxide (ZnO), zinc magnesium oxide (ZnMgO), and the like. Of course therecan be other variations, modifications, and alternatives.

In a specific embodiment, the method includes maintaining the substratemember including the window layer in a second controlled environment 702as shown in FIG. 7. The second controlled environment is characterizedby a relative humidity of less than 50 percent and preferably less than10 percent in a specific embodiment. In a specific embodiment, arelative humidity greater than about 10 percent is characterized by awater vapor species that has a high chemical activity. The secondcontrolled environment is further characterize by a temperature rangingfrom about 10 Degree Celsius to about 40 Degree Celsius in a specificembodiment.

Referring to FIG. 7, the method includes maintaining the substrate in asecond controlled environment 702 after forming the window layer. Thesecond controlled environment can be a desiccated environmentcharacterized by a relative humidity of less than 50 percent andpreferably less than 10 percent in a specific embodiment. In a specificembodiment, a relative humidity greater than about 10 percent ischaracterized by a water vapor species that has a high chemicalactivity. The second controlled environment is further characterized bya temperature ranging from about 10 Degree Celsius to about 40 DegreeCelsius in a specific embodiment. The second controlled environment maybe provided by a dry box using a purging gas such as dried nitrogen orother inert gases, but can be others. Depending on the embodiment, thesubstrate may be stored in a vacuum environment. Keeping the substratein the desiccated environment between processes, for example, afterabsorber layer deposition and window layer deposition, allows animproved performance including higher conversion efficiency of thephotovoltaic cell, which will be further described in more detailed. Ofcourse there can be other variations, modifications, and alternatives

As shown in FIG. 8, the substrate is removed from the second controlledenvironment and a second controlled environment and a second electrodelayer 802 is formed overlying the window layer, forming a photovoltaiccell. The second electrode layer is preferably a transparent conductiveoxide (TCO) to allow a higher fill factor for the photovoltaic cell in aspecific embodiment. The second electrode layer can be a single layerelectrode structure or a multilayer electrode structure. For example,the second electrode layer may use a zinc oxide material having ann-type impurity characteristic. In a specific embodiment, the n-typeimpurity characteristic can be provided using impurity species such asboron, aluminum, indium, and other electron deficient elements. In otherembodiment, the n type impurity characteristic for zinc oxide materialmay be provided using a non-stochiometric zinc oxide such as a zinc richzinc oxide material. The zinc oxide material may be deposited using ametalorganic chemical vapor deposition process in a specific embodiment.Of course there can be other variations, modifications, andalternatives. The second electrode layer may use other transparentconducting material such as indium tin oxide (ITO), fluorine doped tinoxide, and others.

The method includes storing the substrate in a third controlledenvironment t 902 as shown in FIG. 9. The third controlled environmentmay be provided using a dry box using a purging gas such as driednitrogen or other inert gases. Depending on the embodiment, thesubstrate may be stored in a vacuum environment. In a specificembodiment, the third controlled environment is maintained at a relativehumidity of less than 50 percent and preferably less than 10 percent. Bymaintaining the substrate in a controlled or a desiccated environmentbetween processes prior to lamination, the photovoltaic cell can have animproved overall performance such as higher conversion efficiency in aspecific embodiment.

Referring to FIG. 10, the photovoltaic call is sealed using a laminationprocess 1002 to form a solar cell module. The lamination process uses aco-polymer laminating material 1004 such as ethyl vinyl acetate,commonly known as EVA to protect the photovoltaic cell fromenvironmental elements in a specific embodiment. In a specificembodiment, the solar cell module is subjected to a light soakingprocess 1006 to change a conversion efficiency characteristic of thephotovoltaic cell. The light soaking process includes exposing the solarcell module to sunlight or sunlight equivalent for a period of time. Theperiod of time can be less than about 24 hours and may be less than 10hours in a specific embodiment. The light soaking process causes thephotovoltaic cell to change from a first conversion efficiency level toa second conversion efficiency level. In the case of the CIS or the CIGSbased thin film photovoltaic cell, the second conversion level isgreater than the first conversion efficiency level. Depending on theembodiment, the second conversion efficiency level may be one percentagepoint or more greater than the first conversion efficiency level. Thisis in contrast to a silicon based photovoltaic cell where the conversionefficiency tends to decrease with the light soaking process.Additionally, the period of time for a CIGS or CIS based thin filmphotovoltaic cell that is not desiccated after forming the window layerrequires a longer period of light soaking, typically longer than 30hours, to achieve a similar increase in conversion efficiency. Of coursethere can be other variations, modifications, and alternatives.

Depending on the embodiment, there can be variations. For example, themethod can perform other steps such as isolation of the second electrodelayer, electrode bus preparation, perimeter film deletion, ribbonattachment, circuit testing, and others before performing the laminationprocess. Depending upon the embodiment, one or more of these steps canbe added, removed, or others can be added or removed. Of course therecan be other variations, modifications, and alternatives.

Although the present invention has been described using specificembodiments, it should be understood that various changes,modifications, and variations to the method utilized in the presentinvention may be effected without departing from the spirit and scope ofthe present invention as defined in the appended claims. Additionally,although the above has been generally described in terms of a specificstructure for CIS and/or CIGS thin film cells, other specific CIS and/orCIGS configurations can also be used, such as those noted in U.S. Pat.No. 4,612,411 and U.S. Pat. No. 4,611,091, which are hereby incorporatedby reference herein, without departing from the invention described bythe claims herein.

What is claimed is:
 1. A method for processing a thin film photovoltaicmodule, the method comprising: subjecting a plurality of substrates to adesiccated environment having a relative humidity of less than about10%, wherein each substrate of the plurality of substrates includes anabsorber layer overlying a first electrode, and wherein the absorbercomprises a plurality of sodium bearing species having a sodiumconcentration of at least about 5×10¹⁹ atoms per cm³; subjecting theplurality of substrates to a liquid comprising water at a temperaturefrom about 10 degrees Celsius to about 80 degrees Celsius to process theplurality of substrates; subjecting the plurality of substrates to anenvironment having a relative humidity of greater than about 10% to atime period of less than four hours; and after the step of subjectingthe plurality of substrates to a liquid, forming a window layer over theabsorber layer.
 2. The method of claim 1 further comprising performing alamination process on at least one of the plurality of substrates in amodular form as a solar module, the solar module being characterized byan efficiency parameter at a first efficiency level before thelamination process and a second efficiency level after the laminationprocess, the first efficiency level being within a predetermined amountof the second efficiency level.
 3. The method of claim 1 wherein therelative humidity of greater than 10% is characterized by a water vaporspecies having a high chemical activity.
 4. The method of claim 2further performing a light soaking process on the solar module toincrease an efficiency level from a third efficiency to a fourthefficiency.
 5. The method of claim 4 wherein the light soaking processcomprises subjecting the solar module to electromagnetic radiationderived from a solar or a solar equivalent light source.
 6. The methodof claim 4 wherein the light soaking process is maintained for aboutthirty hours or less.
 7. The method of claim 2 wherein the secondefficiency level is greater than the first efficiency level by at least1 percent point.
 8. The method of claim 2 wherein the second efficiencylevel is greater than the first efficiency level by at least 2 percent.9. The method of claim 1 wherein subjecting the plurality of substratesto a liquid includes forming a window layer overlying the absorberlayer.
 10. The method of claim 9 wherein the window layer comprises acadmium sulfide material.
 11. The method of claim 1 wherein the liquidcomprises a cadmium species, an organosulfur species, and an ammoniumspecies provided in a bath maintained at a bath temperature ranging fromabout 50 degree Celsius to about 60 degree Celsius.
 12. The method ofclaim 1 further comprises a patterning process after forming the windowlayer.
 13. The method of claim 12 wherein the patterning process is amechanical patterning process removing selected portions of the absorberlayer material and the window layer material.
 14. The method of claim 1further comprising forming a second electrode layer overlying the windowlayer, the second electrode layer comprises a transparent conductiveoxide selected from: ITO, SnO:F or ZnO:Al.
 15. The method of claim 14further comprises patterning the transparent conductive oxide layer. 16.A method for processing a thin film photovoltaic module, the methodcomprising: subjecting a substrate to a desiccated environment having arelative humidity of less than 10%, wherein the substrate includes anabsorber layer overlying a first electrode, and wherein the absorbercomprises a plurality of sodium bearing species having a sodiumconcentration of at least about 5×10¹⁸ atoms per cm³; subjecting thesubstrate to a liquid comprising water at a temperature from about 10degrees Celsius to about 80 degrees Celsius to process the plurality ofsubstrates after formation of the absorber layer; subjecting thesubstrate having the absorber layer to an environment having a relativehumidity of greater than about 10% to a time period of less than fourhours; and performing a lamination process on at least the substrate toform a solar module, the solar module being characterized by anefficiency parameter at a first efficiency level before the laminationprocess and a second efficiency level after the lamination process. 17.The method of claim 16 further performing a light soaking process on thesolar module to increase its efficiency level.
 18. The method of claim17 wherein the light soaking process comprises subjecting the solarmodule to electromagnetic radiation derived from a solar or a solarequivalent light source.
 19. The method of claim 16 wherein the liquidcomprises a cadmium species, an organosulfur species, and an ammoniumspecies provided in a bath maintained at a bath temperature ranging fromabout 50 degree Celsius to about 60 degree Celsius.
 20. The method ofclaim 16 wherein the first efficiency level is less than the secondefficiency level by at least 1 percent point.